An intra-erythrocytic viral infection, termed piscine erythrocytic necrosis (PEN), which results in massive red blood cell destruction has recently been discovered in the Atlantic cod (Gadus morhua). Evidence has been accumulated that suggests that other marine species, including the Atlantic herring (Clupea harengus), may also be infected with a similar virus. PEN may be a factor influencing fluctuations in the size of populations of these commercially valuable fish. As such, it appears to offer a unique opportunity to study virus-host relationships in the marine environment, a field of investigation that is largely unexplored. PEN virus may also serve as a useful model virus for studying the mechanism of erythrocytic infection with implications for other viruses believed to be involved in hemolytic or hematopathological conditions. This investigation will determine the incidence and geographical distribution of PEN in cod populations in the western Atlantic Ocean. Hematological and other parameters will be monitored on uninfected, infected and experimentally infected cod to determine possible deleterious effects of the infection. PEN virus will be isolated from infected fish and preliminary characterization initiated. Possible in vitro techniques for the isolation and study of the virus will be explored. In addition, erythrocytic degeneration in herring will be studied in greater detail to ascertain if this condition results from a viral infection similar to PEN in cod. Other marine species will also be examined for similar blood lesions. This investigation is a joint effort by the Department of Marine Resources, State of Maine, and the Migratory Fish Research Institute of the University of Maine at Orono, Maine.